Spectroscopic method and apparatus for measuring optical radiation

ABSTRACT

The intensity 2 of radiation coming from an object 1 to be measured and illuminated by collimated radiation is measured at several wavelengths by focusing the radiation, via a lens 4 and a planar mirror 6, on a detector group 3 having several detector elements (3a to 3d). The radiation coming from the object is directed by the lens and the mirror such that, by rotating the mirror, tilted at a small angle with respect to its axis 5, around this axis and by keeping the radiation on the surface of the mirror, the focus F of radiation is moved in a detector plane D along an uninterrupted circular path R crossing each detector element positioned substantially in the same plane and excited at a different wavelength.

BACKGROUND OF THE INVENTION

The invention relates to a method of measuring optical radiation, inwhich method the intensity of a radiation coming from an object to bemeasured and to be lighted by collimated radiation is measured atseveral wavelengths by focusing the radiation by an optical means and amirror means on a detector group comprising several detector elements.

Spectroscopic measurements are based on an investigation of a spectrumof radiation coming from an object to be measured. It depends on theproperties of the object to be measured how the spectral distribution ofthe radiation, which is reflected or emitted from the object or whichhas penetrated the object, is within a frequency range. For instance,the thickness of a film of slushing oil or impurities on the surface ofthe object influences the distribution and amplitude of the spectrumreflected from the object or, in practice, from the film on the surface.

Typical of spectroscopic measurements is that the radiation obtainedfrom a spot-like light source is collimated, i.e. made parallel, and theobject to be measured is lighted by this radiation.

Detectors used in spectroscopic measurements comprise several detectorelements connected to each other, each detector element measuringradiation coming from an object within different wavelength ranges. Fora successful spectroscopic measurement, each detector element should seethe object at the same incidence angle, in order that the multichanneldetector formed by the detector elements may be used for measuringrelative intensities of different wavelengths of the spectrum of theradiation reflected. In practice, it is, however, necessary to positionthe detector elements relatively far from each other for structuraltechnical reasons, due to which the detector elements see the sameobject at slightly different angles or the radiation comes to theseparate detector elements at the same angle, but has started fromanother part of the object than the radiation arrived at the rest of thedetector elements. These problems appearing as error factors inmeasurements cannot be eliminated by fixed optics.

To eliminate these problems, it is known to use a diffuser in front ofthe detector elements. A parallel radiation bundle coming to thediffuser is diffused in the diffuser, which leads to that the radiationcoming to the elements is mixed in different directions. On account ofthe operating principle of the diffuser, a large part of the radiation,i.e. of the light, passes the detectors, and the diffuser attenuatesalso the intensity of the radiation, because the more light rays come tothe diffuser, the more the radiation is attenuated.

U.S. Pat. No. 4,792,684 discloses a horizontal scanner, which is usedfor instance in satellites or missiles to follow their movements. Thedevice described in this publication directs the light rays collected bythe device from several different directions and objects to one detectorelement. By this solution, it is not possible to avoid dimensionalerrors, because the light rays come from different places, due to whicha ratio measurement taking place within a certain wavelength range wouldgive somewhat erroneous measurement results. The mirror means to be usedin the solution according to this publication comprises two separatelevels, which are positioned at an angle with respect to each other. Bymeans of this solution, it is not possible to circulate a focus on adetector level formed by several detectors. In this solution, moreover,the reflecting mirror is tilted to form a big angle of about 30° inrelation to a level perpendicular to its rotating axis. The so-calledsplit mirror structure causes a halving of the light intensity into twoseparate radiation beams.

U.S. Pat. No. 4,748,329 describes a method of and a system for measuringthe thickness of a light transmitting film, whereby a multichanneldetector measures radiation reflected from the surface. Severalreflector means are used in this solution, by means of which the focusof radiation cannot be circulated by one mirror means on the samedetector level over the separate detector elements.

German Patent 36 37 125 discloses a device for measuring reflection tobe used in a spectrometer. In this device, reflection is measured atseveral different incidence angles and a corner prism included in thedevice returns the reflected light in the same direction, and then thelight goes to a detector irrespective of a change in the incidence angleof the reflected radiation. By means of the simple device structure ofthe solution in question, it is not possible to provide a direction ofradiation to a multi-channel detector in such a way that each detectorelement would see the radiation come to the detector elements at thesame incidence angle with the same input aperture.

U.S. Pat. No. 4,923,263 discloses an optic scanner comprising tworotating mirrors tilted with respect to the rotating axis thereof, whichscanner additionally comprises a field lens and a relaying transmissionlens. The mirrors rotate at different speeds and at different phases,due to which several scanning figures of different shapes are obtainedon the detector in the focal plane. This solution concerns a device withan operation similar to that of a camera, in which an image is producedon a first lens already and after that the image is transferred throughmirrors to one and only detector. This solution does not show a focusingof a parallel radiation bundle as late as in the focal plane and analternating circulation of the focus path created in this way overdetector elements excited to different wavelengths, but a transfer of animage produced already earlier to the detector plane, i.e. to the focalplane. The dimensions of the image can be changed by means of adouble-mirror structure.

U.S. Pat. No. 5,089,908 discloses an optic scanning system, by means ofwhich a so-called plywood effect at image production shall beattenuated. The system comprises laser diodes at different wavelengthsoperating according to a control based on a video signal, the radiationof which diodes is directed through optics via a rotating polygonreflector over the detector plane. However, the structure of therotating polygon reflector is in this solution such that it causesdiscontinuities in a light ray and losses in the radiation effectthereof, because the polygon reflector comprises numerous mirrorsurfaces, which, each in turn, reflect the radiation together with therotating movement to the detector, whereby, when moving from one mirrorsurface to another, the radiation is interrupted and the focus ofradiation returns to its starting point. This structure is suitable forimage production for instance in copying machines, but not forspectroscopic applications. Also the basic structure of this systemdiffers considerably from the present solution, for the system comprisesseveral laser diodes operating at different wavelengths, and the systemdoes not even comprise an object to be measured between the light sourceand the focusing optics and the rotating mirror.

Moreover, the publications WO 90/07697, U.S. Pat. Nos. 3,523,734,4,687,329 and 5,050,991 disclose spectrometric methods and devices, inwhich radiation is deflected by means of a grid dispersing radiation,whereby the grid spreads the spot-like radiation to a line spectrum atthe same time to the detectors, which is, however, no good solution inall applications.

Consequently, the solutions according to the prior art comprise a numberof problems. The object of this invention is to set forth a novel methodavoiding the problems associated with the known solutions.

SUMMARY OF THE INVENTION

This object is achieved by means of the method according to theinvention, which is characterized in that the radiation coming from theobject to be measured is directed by the optical means and the planarmirror means in such a way that, by rotating the mirror means tiltedwith respect to its axis round this axis and by keeping the radiation onthe surface of the same planar mirror means, the focus of radiation ismoved in a detector plane along a regular uninterrupted pathalternatingly over each detector element positioned substantially in thesame plane and excited at a different wavelength.

The method of the invention is based on the idea that the radiationcoming from the same part of the object to be measured is brought at thesame solid angle and with the same input aperture to the separatedetector elements, without any substantial loss of radiation intensity,however.

Several advantages are achieved by the method of the invention. Theelements of the detector see the object exactly at the same solid angleand at the same place, due to which the dimensional errors areeliminated. The device structure required for the realization of themethod has a Good optical efficiency and is relatively simple and lightand thus also safe to operate and durable, which is a special advantagein connection with portable measuring devices.

The invention also relates to a device for measuring optical radiation,which device comprises a means for lighting an object to be measured bysubstantially parallel radiation, an optic focusing means for directingthe radiation coming from the object to be measured and a planar mirrormeans rotatable round its rotating axis for deflecting the radiation tobe focused to a detector group comprising several detector elements. Thedevice is characterized in that the optic focusing means directingradiation is arranged between the object to be measured and the planarmirror means in such a way and at such a distance from the mirror meansthat the radiation coming from the optic focusing means is focusedduring the whole measurement through the same plane surface of theplanar mirror means at least approximately to the detector plane andthat the planar mirror means between the optic focusing means and thedetector group deflecting the radiation to be focused to the detectorgroup is tilted to a small angle with respect to a plane rectangular toits rotating axis in such a way that the focus of the radiation can bemoved by means of the mirror means rotatable round its rotating axis ina detector plane formed by the detector elements situated substantiallyin the same plane alternatingly over the separate detector elements.

The device according to the invention permits a radiation coming fromthe same part of the object to come at the same solid angle and with thesame input aperture to all separate detector elements situated in thesame detector plane. The optical efficiency of the device of theinvention still remains sufficiently high. The device of the inventionremains mechanically simple, which is a remarkable advantage inconnection with portable measuring devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following in greater detail withreference to the enclosed drawings, in which;

FIG. 1 shows general diagram of a device solution to be used in themethod,

FIG. 2 shows the general diagram of the device solution to be used inthe method seen towards a detector plane,

FIG. 3 show a broadened path along which a focus rotated in detectorplane circulates,

FIG. 4 shows a surface roughness formed on the surface of a mirrormeans, and

FIG. 5 shows another preferred embodiment of the device of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2, a device solution according to the basic idea of theinvention is used for instance in a spectrometric measurement formeasuring an optical radiation 2 coming from an object 1 to be measured.The device comprises a multi-element detector group 3, i.e. a detectorgroup comprising several detector elements 3a, 3b . . . , an opticalmeans 4 for directing radiation and a mirror means 6 rotatable round itsrotating axis 5 for deflecting radiation. The optical means 4 directingradiation can be used a convex lens, a lens combination, or a mirror.According to the inventive basic idea of the device solution, theoptical means 4 directing optical radiation 2 is positioned between theobject 1 to be measured and the mirror means 6 at such a distance fromthe mirror means 6 that the optical radiation 2 coming through themirror means 6 to the detector group is focused through the mirror means6 at least approximately in a detector plane D, and that the mirrormeans 6 between the optical means 4 and the detector group 3 is tiltedto form a small angle α in relation to a plane perpendicular to itsrotating axis 5 in such a way that a focus F of radiation can be movedin the detector plane D formed by the detector elements 3a to 3dsituated substantially in the same plane over the separate detectorelements 3a to 3d by means of the mirror means 6 rotatable round itsrotating axis 5. On FIG. 2 can be commented that, for the sake ofclarity, the detector group 3 and its detector plane D are shownslightly tilted backwards, in order that the movement of the focus F inthe detector plane D could be seen more clearly.

The very focus P of the optical means 4, e.g. of a lens, is situated atthe distance of a focal length f from the optical means 4. The opticalmeans 4 is positioned at such a distance x from the mirror means 6 thatthe total of said distance x and a distance y between the detector planeD and the mirror means 6 is at least approximately equal to the focallength f of the optical means.

According to FIGS. 1 and 2, the optical radiation required for ameasurement and to be directed to the object 1 to be measured and to afilm 7 on its surface is provided either by means of an external lightsource 200 or a light source (not shown) included in the measuringdevice itself. The radiation 205 obtained from the spot-like lightsource 200 is made parallel, i.e. collimated, by a collimator lens 210.

In a preferred embodiment of the device, the mirror means 6 tilted toform a small angle α consists of a substantially integral aluminiumpiece, which makes the mirror means 6 light and advantageous and simpleto manufacture. The lightness permits the mirror to be rotated by asmall electric motor 8, the power consumption of which is only of theorder below 4 mA. In a practical realization, the weight of the mirrormeans manufactured of aluminium was only about 3 grams. The machiningproperties of the mirror means of aluminium are relatively good.

In a preferred embodiment of the device of the invention, the device canadditionally comprise a diffuser means consisting of a surface roughness9 formed on the surface of the mirror means 6 according to FIG. 4. Ithas been possible to combine the diffuser means realized in this waywith the rest of the device structure, i.e. in this case with thesurface of the mirror means 6, in a compact manner. The abovearrangement is very durable and usable, compared to the use of adiffuser lens 20. The surface of the mirror means 6 made of aluminium orsome other material can be provided with the surface roughness 9 eitheras a separate stage of operation or by leaving the reflecting surface ofthe mirror means 6 slightly deficiently polished or possibly a littlerough or dull. The diffuser means realized in this way, which in thiscase preferably means the surface roughness 9 of the mirror means ofaluminium, provides a broadening of a path R, along which the focuscirculates on the detector level D, to a breadth a or width "w" as shownin FIG. 3, which is thus much broader than the breadth of a thinnercircular path R shown in FIG. 2. A diffuser means of this kind isneeded, if there are surface defective areas or for instance dirt on thedetector elements 3a to 3d. A broadened focus F attenuates the influenceof such spotlike disturbances on the measurement result. FIG. 1 shows adiffuser, such as a diffuser plate 20, positioned directly on theoptical path in front of the optical means 4, due to which theadjustability of the diffuser means can be implemented more easily.

In a preferred embodiment of the device solution, the tilt angle α ofthe mirror means with respect to a level perpendicular to the axis 5thereof is smaller than 10°, preferably approximately 6°. Then the focusF can be circulated along the regular symmetric path R, but, however, ata sufficiently small incidence angle with respect to the detector planeD, in order that a use of interference filters (not shown) positioned infront of the detector elements 3a and 3b would be possible. The use ofinterference filters requires a sufficiently small incidence angle ofradiation in relation to the detector plane D. Optical means of otherkinds, such as polarizers, can also be used in front of the detectors.

The invention relates also to a method of measuring optical radiation 2coming from an object 1 to be measured, for instance in a spectrometricmeasurement. Then by using the device solution according to theinvention, the radiation 2 coming from the object 1 to be measured ismeasured in the method by focusing the radiation by an optical means 4and a mirror means 6 on a detector group 3 comprising several detectorelements. According to the basic idea of the method of the invention,the radiation 2 coming from the object 1 to be measured is directed bythe optical means 4 to the mirror means 6, and a focus F of radiation ismoved by rotating the mirror means 6 tilted with respect to its axis 5round this axis 5 on a detector plane D along a regular path R overdetector elements 3a to 3d positioned substantially in the same plane.Then the optical radiation coming from the object 1 to be measured isformed as a continuous radiation bundle, which is focused on thedetector plane D. In a preferred embodiment of the method of theinvention, the focus F is moved in the detector plane D over thedetector elements 3a to 3d along a substantially circular, ellipsoidalor otherwise continuous path. Then the path of movement of the focus Fis such that it can be focused on the detector plane D by means of asimple device solution. Referring to FIG. 3, if it were desirable tobroaden the focus F of radiation for instance on account of impuritieson the detector elements, then the focus F is broadened in the detectorplane D by the aid of an integral diffuser means on the mirror means 6,which diffuser means is preferably a surface roughness 9 formed on thesurface of the mirror means. In FIG. 3, the broadening of the focus isto be seen as an increase of a breadth "a" of the path R, along whichthe focus F circulates.

According to FIG. 2, the optical radiation to be focused by the opticalmeans 4 is deflected by the mirror means 6 to the detector plane D, inwhich the focus F of the optical radiation is moved on the basis of therotation of the mirror means 6 in turn over the separate detectorelements 3a to 3d. FIG. 2 shows two conical bundles of radiation, i.e.radiation beams 10 and 11, tapering conically towards the detector, thelatter one of which is indicated by broken lines. Respectively, twodifferent positions are presented also for the mirror means, of whichpositions the one producing the beam 11 is also indicated by brokenlines. The difference between the positions of the mirror means is 180°,i.e. the distance of half a turn round the axis 5 of the mirror means 6.The radiation beams 10 and 11 illustrate the focusing of a conicallytapering radiation beam on the detector plane D at various moments oftime. Thanks to the rotation of the mirror means 6 and the focusinginfluence of the optical means 4, e.g. a convex lens, the focus F ofradiation can be moved easily in the same detector plane D over theseparate detector elements 3a to 3d.

FIG. 5 shows another preferred embodiment of the device of theinvention, in which fiber optic photoconductors 23a to 23n are connectedto the actual detector elements 3a to 3n. Then in practice, the detectorplane D will be a plane formed by ends 24a to 24n of the photoconductors23a to 23n and the detector group 3 is formed by the actual detectorelements 3a to 3n and the photoconductors 23a to 23n. As far as theterminology associated with detectors is concerned, the photoconductors23a to 23n connected to the actual detector elements 3a to 3n shall beconsidered to be functionally included within the term "detectorelement". As a photoconductor can serve for instance an optical fibre ora bundle of optical fibres. By means of the solution of FIG. 5, thedetector plane D itself can be made small in size, for the detectorplane is formed by the ends 24a to 24n of the photoconductors 23a to 23npositioned in front of the detectors. By means of the structure of FIG.5, it is possible to collect radiation to several optical fibres exactlyat the same incidence angle, with the same aperture and from the sameobject. The fibre ends can be preferably positioned in a circular formaccording to the path R of the focus, whereby the collecting ratio oflight is optimized.

In front of the detectors 3a to 3n, it is also possible to connect othermeans, such as wavelength filters, polarizers and other optical means,which are indicated in FIG. 5 by reference numerals 25a to 25n. Thedetector solution connected to photoconductors according to FIG. 5 isadvantageous also in this case, for the additional other optical means25a to 25n can be positioned between the actual detector elements 3a to3n and the photoconductors 23a to 23n.

In FIG. 5, the measurement environment is presented in such a way thatthe figure shows no surface as an object to be measured from which lightradiation would be reflected. On the contrary, it is a question ofmeasuring the radiation 2 coming from an object to be measured, i.e.from a flue gas duct, for instance. Consequently, the method and thedevice according to the invention are suitable for measuring reflectedradiation as well as radiation coming from the object in some other way.

Though the invention has been described above referring to the examplesof the enclosed drawings, it is clear that the invention is notrestricted to them, but it can be modified in many ways within the scopeof the inventive idea presented in the enclosed claims.

We claim:
 1. A method of spectroscopically measuring optical radiation,in which the intensity of radiation coming from an object (1) to bemeasured and exposed to collimated radiation is measured at a pluralityof different wavelengths by focusing the radiation, via optical means(4) and mirror means (6), on a detector group (3) comprising a pluralityof detector elements (3a to 3d), comprising the steps of:a) orientingthe optical means to direct the radiation to a planar surface of themirror means, said planar surface being tilted at a small acute angle(α) relative to a plane perpendicular to a rotational axis (5) of themirror means, b) rotating the mirror means around said axis such that afocus (F) of the radiation moves in and defines a continuous,repetitious path (R) in a detection plane, and c) disposing the detectorgroup with detection surfaces of the detector elements lying in saiddetection plane and in said path, such that the moving radiation focusrepeatedly sweeps across the detection surfaces of the elements.
 2. Amethod according to claim 1, wherein said path (R) is substantiallycircular or ellipsoidal.
 3. A method according to claim 1, wherein thefocus (F) is broadened in the detector plane (D) by integral diffusermeans (9) on the mirror means (6).
 4. A method according to claim 1,wherein the optical means comprises a focusing lens.
 5. A device forspectroscopically measuring optical radiation, comprising:a) means (200,210) for lighting an object 1,7) to be measured with substantiallyparallel radiation, b) optical focusing means (4) for directingradiation (2) coming from the object to be measured, and c) planarmirror means (6) rotatable around an axis thereof for deflecting theradiation to a detector group (3) comprising a plurality of detectorelements (3a to 3d), wherein d) the focusing means (4) is disposedbetween the object and the mirror means (6) such that the radiationcoming from the optical focusing means is reflected by the planarsurface of the mirror means and focused in a detection plane (D), e) theplanar surface of the mirror means is tilted at a small acute angle (α)with respect to a plane perpendicular to an axis (5) thereof, and f) themirror means (6) is rotatable around said axis such that detectionsurfaces of the detector elements (3a to 3d) situated in the detectionplane are repeatedly and sequentially swept by the focus (F) of theradiation moving in a continuous, repetitious path (R).
 6. A deviceaccording to claim 5, wherein the mirror means (6) comprises asubstantially integral aluminium member.
 7. A device according to claim5, further comprising diffuser means defined by roughness (9) formed onthe surface of the mirror means.
 8. A device according to claim 5,wherein the tilt angle (α) of the mirror means surface is less than 10°,preferably approximately 6°.
 9. A device according to claim 5, whereinsaid path (R) is substantially circular or ellipsoidal.
 10. A deviceaccording to claim 5, wherein said detection surfaces of the detectorelements are defined by ends (24a-24n) of fiber optic members.